[?] 04 Deville DTS w/ Bose Speaker Issues

When taking on a 04 Deville DTS w/ Bose after cleaning out the TB, I was amazed on how bad the sound was compared to an old 99 Seville that I personally owned not too long ago. Upon closer observation, we learned that the rear passenger speaker is inoperable, the front passenger speaker produces distorted output, and the sub in the back is producing very little output. When carefully touching the membrane of the sub you feel it's moving a little bit but nothing in comparison to any normal speaker.

I have read through countless audio threads here but was unable to locate a simple step by step diagnose procedure. Do I have to take all doors apart or is there a single spot that should be my starting point? Is diagnose as easy as using a multi-meter to measure the signal strength before and after the speaker?